An FET (Field Effect Transistor) using compound semiconductors, such as GaN, has outstanding high frequency characteristics, and is widely put in practical use as a semiconductor device which operates at a microwave band.
This FET has the following structures, for example. That is, a GaN buffer layer is formed on a SiC substrate, and an undoped AlGaN layer which is an active layer is formed on the GaN buffer layer. On the undoped AlGaN layer, a drain electrode and a source electrode which consist of Ti/Al/Ni/Au are formed. The drain electrode and the source electrode form ohmic junctions with the undoped AlGaN layer. The drain electrode and the source electrode are formed by laminating Ti/Al/Ni/Au and removing an unnecessary portion by a lift-off method. Furthermore, between the drain electrode and the source electrode, a gate electrode which consists of Pt/Au or Ni/Au is formed. The gate electrode forms a Schottky barrier junction with the undoped AlGaN layer. The gate electrode is formed by laminating Pt/Au or Ni/Au and removing an unnecessary portion by the lift-off method.
Since a layer of a GaN system is a semiconductor layer which is bad in reactivity with Pt or Ni, in the FET mentioned above, adhesion of the AlGaN layer and Pt or the AlGaN layer and Ni is not so good. For this reason, when forming the gate electrode by the lift-off method, there was a problem that the formed gate electrode was peeled off.
In order to avoid this, a method of improving the adhesion of a gate electrode and a semiconductor substrate by forming the gate electrode with a vapor deposition rate of 2-20 Å/second is known (Japanese patent application publication No. HEI 10-178189).
When a gate length is shortened in order to make the FET highly efficient, a bonded surface of the AlGaN layer and the gate electrode becomes small. For this reason, there is a problem that it becomes difficult to acquire sufficient adhesion also by the method mentioned above.